Don't usually read fanfic anymore, most of the stuff I've read tended to focus on the fanfic author's favorite traits of a particular character, rather than exhibiting the depth that the originating author would instill. Having said that, however, I do recall a series of fantasy novels that were set in a particular city. I forget (gasp!) the original author, or the names of any of the novels, the characters, or even the city. Something quite similar to Terry Pratchett's Discworld characters, but waaaay before him (yeah, I've got some years on me.).
Anyway, after the originator of the series (so it appeared) started to run out of stories to tell, somebody came up with the idea of the shared universe, in which different authors would borrow some of the original characters and use them in a supporting role to their own (newly-minted) protagonist; again, set within the same city. This was done with the original author's consent and, sometimes, complicity - as in co-writer. As I recall the main ground rules were that the borrowing author could not kill off an original character, nor have the original character exhibit behavior which was grossly "out of character".
Quite ofter the non-originating writers would say in the author's notes that they were raving (sic) fans of the original works, and were so very, very honored to be allowed to play in that universe.
So, on the whole 'fanfic-angel or demon' question, I'm with the pitchforks and torches crowd, except when...